Container Check Digit Calculator
Answer: A container check digit is calculated from the first 10 ISO 6346 characters by converting letters to numeric values, multiplying each position by powers of two, and taking the weighted sum modulo 11.
Paste a full ISO 6346 number, or edit the split fields below for manual checking.
MSKU123456 uses ISO 6346 letter values, weights 1, 2, 4, 8, 16, 32, 64, 128, 256, and 512, then sum modulo 11. The weighted sum is 5560; 5560 mod 11 = 5; the check digit is 5.
| Position | Char | Value | Weight | Product |
|---|---|---|---|---|
| 1 | M | 24 | 1 | 24 |
| 2 | S | 30 | 2 | 60 |
| 3 | K | 21 | 4 | 84 |
| 4 | U | 32 | 8 | 256 |
| 5 | 1 | 1 | 16 | 16 |
| 6 | 2 | 2 | 32 | 64 |
| 7 | 3 | 3 | 64 | 192 |
| 8 | 4 | 4 | 128 | 512 |
| 9 | 5 | 5 | 256 | 1280 |
| 10 | 6 | 6 | 512 | 3072 |
| Weighted sum | 5560 | |||
A container check digit calculator computes the final ISO 6346 digit for a container number by weighting the first 10 characters and taking the sum modulo 11.
How the calculation works
An ISO 6346 container number is built from a three-letter owner/operator code, one equipment category letter, a six-digit serial number, and one check digit. For example, MSKU1234565 breaks into owner code MSK, category U, serial 123456, and check digit 5.
The calculation converts letters into ISO 6346 numeric values where A=10, B=12, C=13, and the sequence skips multiples of 11. Digits keep their face value. Each of the first 10 characters is multiplied by a positional weight: 1, 2, 4, 8, 16, 32, 64, 128, 256, and 512. Add those products, divide by 11, and use the remainder as the check digit. If the remainder is 10, the printed check digit is 0.
Freight operators use this as a quick data-quality check before sending container IDs into booking, bill of lading, customs, terminal, EDI, or shipment-tracking workflows. A valid check digit catches many transcription errors, but it does not prove the container exists, is released, or is currently at a terminal.
FAQ
What is a container check digit?
It is the final digit in an ISO 6346 container number. It is calculated from the first 10 characters so systems can detect common typing mistakes.
What format should I enter?
Enter three owner-code letters, one category letter, and six serial digits, such as MSKU123456. You can also paste a full 11-character number such as MSKU1234565 to validate the supplied digit.
What does the U in a container number mean?
U is the common ISO 6346 equipment category for freight containers. J is used for detachable equipment, and Z is used for trailers or chassis.
Can the calculator accept spaces or hyphens?
Yes. Inputs such as MSKU 123456-5 are normalized before validation, which matches how freight teams often receive container IDs in emails or spreadsheets.
Why does ISO 6346 skip some letter values?
Letter values skip 11, 22, and 33 so the modulo 11 arithmetic works consistently across alphabetic and numeric characters.
Why does a remainder of 10 become check digit 0?
The final printed check digit must be a single decimal digit. ISO 6346 therefore represents a modulo remainder of 10 as 0.
Does a valid check digit mean the container is real?
No. A valid result only confirms the number passes the arithmetic check. It does not confirm BIC registration, carrier ownership, availability, tracking status, or customs release.
When should a forwarder check the digit?
Check it before submitting shipping instructions, issuing draft bills of lading, creating EDI messages, sending customer updates, or reconciling terminal and carrier tracking events.
Comparison
| Check method | Best for | What it catches | Limitation |
|---|---|---|---|
| ISO 6346 check digit calculator | Fast freight-ops validation before data entry | Many mistyped owner codes, serial digits, and copied numbers | Does not confirm live container status |
| Carrier or terminal tracking portal | Confirming movement milestones and location | Invalid numbers rejected by a specific carrier system | May require the correct carrier, booking, or bill of lading context |
| BIC owner-code lookup | Checking registered owner-code legitimacy | Unknown or suspicious owner/operator codes | Does not calculate the serial check digit |
| Manual spreadsheet validation | Bulk cleanup of container lists | Repeated formatting and check-digit errors | Easy to misconfigure without the exact ISO 6346 formula |
Dated facts
As of May 13, 2026, ISO 6346 remains the standard structure used for intermodal freight container identification: three owner-code letters, one equipment category letter, six serial digits, and one check digit.
As of May 13, 2026, U remains the usual category identifier for freight containers, while J is used for detachable equipment and Z for trailers or chassis.
As of May 13, 2026, a check digit is still only an arithmetic validation signal; freight teams should verify live container events through carrier, terminal, or visibility-platform data before making release, demurrage, or delivery decisions.
Frequently asked questions
- What is a container check digit?
- It is the final digit in an ISO 6346 container number, used to detect typing errors in the owner code, category letter, and six-digit serial number.
- Can this validate a full container number?
- Yes. Paste an 11-character container number such as MSKU1234565 and the calculator compares the supplied final digit with the computed digit.
- What does the U category mean?
- U is the standard ISO 6346 equipment category for freight containers. J is used for detachable equipment and Z for trailers or chassis.
- Why can the remainder 10 become check digit 0?
- ISO 6346 uses modulo 11, but the printed check digit must be one decimal digit, so a remainder of 10 is represented as 0.
- Does a valid check digit prove the container exists?
- No. It only proves the number passes the ISO 6346 arithmetic check; it does not confirm ownership, availability, location, or carrier release.
- Where should freight teams use this?
- Use it before sending container numbers to carriers, terminals, customs brokers, customers, EDI systems, or shipment-tracking workflows.